Process for the preparation of branched conjugated diene polymers



United States Patent US. Cl. 260-784 4 Claims ABSTRACT OF THE DISCLOSUREConjugated diene polymers having a branched molecular configuration areprepared by first polymerizing a conjugated diene with an alkyl lithiuminitiator and thereafter coupling the first polymer with a diester ofdicarboxylic acids with monohydric alcohol to form a low molecularweight polymer having satisfactory coagulation characteristics,driability, and processability.

BACKGROUND OF THE INVENTION Synthetic rubbers made from conjugateddienes have been studied extensively with respect to the polymerizationinitiators or catalysts, the structures of the polymers so obtained andtheir properties relative to natural rubbers. While it is possible tovary the structure of synthetic rubbers by the use of particulartechniques, it is often difficult to obtain a tailor-made polymer havinga desirable combination of characteristics from a manufacturing andprocessing standpoint. It is necessary for the production of low-costpolymers that they be amenable to recovery from their polymerizationmedium, be readily driable and exhibit good processing characteristics.

Processing characteristics during mixing in a Banbury or other internaltype mixer include:

(1) Coherency of Banbury discharge;

(2) Power required to mix; and

(3) Temperature of batch at discharge after a standard mixing cycle.

(1) Coherency of batch.-Discharge should be rapid and in a sequence ofgood sized, smooth, glossy lumps called Whales because of their shape.With natural rubber the ideal lumps are all loosely or lightly connectedin a string on discharge. The discharge should be rapid and entire, Withno difficulty in removing the batch from the Banbury. An undesirablebatch with respect to processability is a fine granular dischargeaccompanied by undispersed black. The transition from one end of thequality scale to the other occurs in subtle stages. If mixed in a testrecipe, linear polypolyisoprene discharges as a mass of fine granulestogether with free black.

(2) Band formation on sheet-of) milL-The Banbury discharge is put ona-mill, and the ease with which a. smooth, coherent, non-lacey,nonrugose (lumpy) sheet is formed is estimated. Only a few (2-4)revolutions of the mill roll are permitted at this stage. After thisevaluation the batch is further milled and evaluated for its millhandling characteristics: how it cuts and folds and how sticky, soft, orstrong it is.

After milling, the stock is remixed in Banbury at about 75 F. toincorporate the curatives, then it is rested overnight and remilled.

The final stock is extruded through a Garvey dieaccording to ASTM-D2230and rated from 1 (poor) to 16 (excellent). Finally, the compound Mooneyviscosity is measured.

(3) Blends.Furthermore, and of substantial importance, polyisopreneshould be capable of being mixed 3,551,392 Patented Dec. 29, 1970 "icewith other elastomers, particularly natural rubber, to form good blendsin a reasonable length of time.

One of the well-known types of polymerization initiators for conjugateddienes is an alkyl lithium. Normally, the alkyl lithium is empolyed toinitiate polymerization of a conjugated diene in a hydrocarbon medium,resulting in the formation of long chains which are essentiallyunbranched; so much so, in fact, that they are referred to as linearpolymers. It has been found that it is necessary to prepare relativelyhigh molecular weight linear polymers of this type if they are to bereadily separated from the polymerization solvent as discrete particlesof rubber, (often referred to as crumb) by the coagulation with steam orhot water.

Furthermore, it has been found in the past that relatively highmolecular weight linear polymers are required if the coagulation crumbis to be dried under elevated temperature conditions such as on a movingbelt through a forced air heating area. If a high molecular weightproduct of linear structure is not utilized, then during coagulation inhot Water and steam, the crumb generally agglomerates to a mass whichcannot be recovered to an econcmically useful product. Moreover, even ifa suitable crumb structure is formed upon coagulation, it has beendetermined that unless the intrinsic viscosity of the linear polymer isin the order of 6 or higher, the crumbs have poor driability, since thenormally porous crumbs tend to fuse at the surface upon heating, sealingWater with this structure.

Finally, the high molecular weight linear polymers which pass thesefirst two criteria are then found to be diificult to process at leastuntil such time as they have been subjected to a sufficient amount ofshear so that their intrinsic viscosity is drastically reduced.Moreover, it has been noted that the relatively high molecular weightlinear polymers blend with reluctance with other rubbers such as naturalrubber and in the course of such blending operations crumbling oftenoccurs and vulcanization recipe components are often poorly dispersed.Loss of ingredients represents an extreme case. It is important thathighly reinforcing carbon blacks (ISAF or HAF) be more than justincorporated. They should be well dispersed. This is observed visuallyas smooth, glossy lumps, The Banbury dumps of linear elastomers arefound to be crumbly in nature and of an unsatisfactory consistency forgood mixing.

Recently, several attempts have been made to alleviate this situation bychanging the basic structure of the lithium alkyl initiated polymerssuch as by the use of so-called branching agents, of which divinylbenzene is regarded as typical; and of certain coupling agents which upto this time have been thought to include only those coupling agentshaving at least three sites capable of reaction with lithium to carbonbonds.

OBJECT OF THE INVENTION It is an object of the present invention toprovide a process for the preparation of conjugated diene polymershaving improved manufacturing characteristics. It is a particular objectof the invention to provide a process for the production of a branchedconjugated diene polymer showing improved coagulation characteristics.It is a further object of the invention to provide a process for theproduction of a branched conjugated diene polymer having satisfactorydriability. Finally, it is an object of the invention to provide aprocess for the production of certain branched conjugated diene polymersshowing improved processing characteristics. Other objects will becomeapparent during the following detailed description of the invention.

STATEMENT OF THE INVENTION Now, in accordance with the presentinvention, a process is provided for the preparation of a branchedconjugated diene homopolymer which comprises polymerizing a conjugateddiene having 4-5 carbon atoms per molecule in the presence of an alkyllithium initiator to form a polydienyl lithium first polymer andreacting the first polymer with 0.5-1.5 equivalents per equivalent oflithium of a monohydric alcohol ester of a dicarboxylic acid, whereby abranched polymer having an intrinsic viscosity between about 2.0 and3.75 is formed. It has been found that the products of this processexhibit in a single individual product the three criteria referred to inthe preceeding paragraph, namely, good coagulation characteristics, gooddriability and good processing characteristics. While the process isespecially designed to prepare a branched polyisoprene elastomer, it maybe also utilized for the formation of branched polybutadiene and SBR.The process especially contemplates the use as the coupling compound ofa diester of a fatty alcohol and an aliphatic dicarboxylic acid.Preferably the branched product has an intrinsic viscosity of 2.2-3.6(preferably 2.6- 3.0) dl./g., solvent: toluene, temperature: 30 C.

The star-shaped polymers of this invention discharge from a standardBanbury test as coherent, smooth, glossy lumps. They exhibit good bandformation on a sheet-off mill, give excellent Garvey rating extrusionsat a high extrusion speed, and show acceptable compound Mooneyviscosities, which are in direct proportion to their raw Mooneys.

The products formed by the use of the diester coupling agents of thisgenerically claimed class have been found to be primarily mixtures oftrimer and tetramer of the initial polymer; the extent of each has notyet been determined. While it is not intended to limit the presentinvention, it is postulated that the following equation represents themajority of the coupling reaction which occurs:

In the above equation the term Et is meant to represent an ethyl radicalwhile the letter P represents the initially formed conjugated dienepolymer terminated with a lithium ion. It will be noted from the aboveequation that the use of the subject class of diesters permits theformation of coupled products having the form of tetramers of the firstpolymer. This is in direct contrast to the results which would beobtained if an ester of a monocarboxylic acid and a polyhydric alcoholwere utilized in place of the class forming the important aspect of thisinvention.

It is stressed that the diesters which are operable in the presentprocess must be those in which the carboxyl radicals of the acid fromwhich the ester is made are directly attached to a carbon atom;preferably the two carboxyls are connected by carbon-to-carbon bondsonly and no carbon-to-oxygen bonds are present in these connectinglinks. However, the acids may contain heteroatoms such as oxygen,nitrogen or sulfur, replacing the carbon atoms in the chain. Thefollowing lists of aliphatic acids illustrate the dicarboxylic acidswhich may be used for the formation of suitable esters.

ALIPHATIC ACIDS Oxalic Maleic Malonic Fumaric Succinic Glutaric AdipicPimelic Suberic Sebacic Itaconic The following list of aromatic acidsillustrate the type of dicarboxylic acids which may be employed forforming suitable esters:

AROMATIC ACIDS Phthalic Isophthalic Terephthalic Naphthalic DiphenicEsters of the above types of dicarboxylic acids may be formed fromeither aliphatic or aromatic monohydric alcohols of which the followingare typical:

MONOHYDRIC ALCOHOLS Methyl Ethyl n-Propyl Isopropyl n-Butyl sec-Butyltert.-Butyl Amyl Hexyl Octyl Phenol Cresol The esters may bear alkyl oraryl substituents without altering the nature of the present invention.The following esters are typical of those prepared from the above typesof acids and esters:

ESTERS Diethyl oxalate Dibutyl glutarate Dimethyl adipate One of theimportant aspects of the present invention is the discovery of theintrinsic viscosity range which will result in polymers of goodprocessability and which at the same time will also exhibit goodcoagulation characteristics and good drying properties. The branchedconfiguration of the subject polymers is required for this combinationof properties in a single product. Furthermore, the relatively lowintrinsic viscosity recited hereinabove is required for goodprocessability but because of the branched configuration this alsopermits good driability and coagulation characteristics. Branchedpolymers obtained by the process of the invention, with the exceptionthat the intrinsic viscosity is permitted to be substantially higherthan that recited, have the same ditficulties of processabilityexperienced with linear polymers of relatively high intrinsic viscosity.Linear polymers which are essentially unbranched and which are ofrelatively low intrinsic viscosity within the range cited, i.e.,2.0-3.75, have apparently good processability but are not capable ofbeing coagulated by hot water or steam and have extremely poor dryingand storage characteristics. Star-shaped polyisoprene having anintrinsic viscosity (IV) of 1.8 dL/g. does not dry well. A 2.2 dL/g. lVpolymer dries but is somewhat sticky when hot and shows some cold flowon storage. The optimum IV is about 2.6- 3.0 dL/g. When the IV gets muchabove 3.6 dl./g. the processability begins to worsen, Banbury dumpsbegin to crumble, extrusion rate, and Garvey ratings decrease, and thecompounded Mooney goes above the desirable range. Consequently, theprocess of the present invention is directed to the preparation ofhomopolymers of conjugated dienes having the restricted intrinsicviscosity range so as to result in the critical combination of threecharacteristics recited hereinbefore.

The intrinsic viscosity of the first polymer is readily controlled bythe proportion of lithium alkyl initiator employed in its formation.Normally for the production of this first polymer between about 0.3 andabout 0.8 milliequivalent per grams of monomer are employed. Thepolymerization is normally carried out in an inert hydrocarbon mediumsuch as in cyclohexane or isopentane or mixtures thereof. Otherrelatively volatile hydrocarbons may be employed in addition to or inplace of these species, the criterion being that the hydrocarbon mediumis one capable. of being flashed off in contactwith hot Water and/orsteam. The process generally comprises admixture of an alkyl lithiumwith conjugated diene monomer in the presence of the inert hydrocarbonmedium and polymerization at temperatures between about 20 C. and about100 C. to result in a first lithium-terminated polymer which usuallywill have an intrinsic viscosity in the order of l.02.5 and preferablywithin the range from about 1.2-2.0.

Following the production of this first polymer, the diester is thenadded to the reaction mixture to cause the production of the coupledbranched polymers desired as the end product of this invention. Theesters are usually added in an amount between about 0.5 and 1.5equivalents izing at the temperatures and times indicated in Table Iwhich follows. The series of polymers prepared were varied in theirfirst polymer, i.e., precoupled intrinsic viscosity so that when coupledwith the selected diester (diethyl adipate) the resulting series ofpolymers would have the spread in final intrinsic viscosities indicatedalso in Table I. It will be noted according to Table I thatpol'ymerization temperatures varied from 4060 C. with reaction timesvarying from about 2 /2 hours to hours and that the time allowed forcoupling after addition of diethyl adipate was about minutes. The cis1,4-content of the coupled products is relatively high, in the order of7080% as determined by nuclear magnetic resonance, and the products hadsatisfactory drying rates.

TABIJE I-POLYISOTRENE POLYMERIZATION AND RECOVERY DATA Dietert CouplingTotal drying Pre- Initiator Solids, Diethyl reaction reaction rate, Re,coupled eonc., percent Temp, adipate/ time, time, 1b./(hr.-tt. I.V. Finap.p.m. wt. C. Li ratio minutes minutes at 180 F. dl./g. I.V

per equivalent of lithium ions. Reaction with the coupling agent isusually rapid but the rate will depend in part upon the temperature,which is preferably between 20 and 80 C., normally between -75 C. Thereaction mixture usually is held only for short periods for coupling,usually about 0.1-4 hours. The coupling process may be carried out in abatch manner or continuously.

Following the coupling reaction, any residual carbon- Premium qualitytread stock 3! ions are terminated by a protonating agent such as by the9 3 100 addition of water, alcohol or other similar reagent. The Zlnc 3product may be recovered by coagulation utilizing hot Swan: acld 3 wateror steam or both to recover the polymer in acrumbphenyl-betanaphthylamlne 1 like product which is then subjected todrying and there- P black 50 after is utilized for various end usesincluding compound- Amman? extendlng 011 5 d ti tread and tire carcassstocks. Usually it will be Sulfur 2 used in conjunction with anotherrubber such as natural N'cyclohexyl'z'benzothlazolesulfenamlde 0.6

A series of polyisoprenes was formed involving polymerization ofisoprene utilizing secondary butyl lithium as the initiator. Thepolymerizations were carried out by mixing isoprene in isopentane as areaction solvent and polymer- It will be noted according to Table IIthat only the two samples A and B having final coupled intrinsicviscosities of 2.2 and 3.6, respectively, showed rapid blackincorporation times, good or fair Banbury dump quality, and excellentGarvey rating as well as high extrusion rates compared with the SamplesC and D which are also branched polymers but have substantially higherintrinsic viscosities. The latter two samples not only crumbled in theBanbury dump and, therefore, were unsatisfactory, but also had farlonger black incorporation times and much poorer Garvey ratings as wellas lower extrusion rates.

TABLE II.P ROCESSING CHARACTERISTICS The Black Banbury dump Garveycoupled Coupled incorp. Compound Extrusion I. .V., time, Temp., Power,Mooney, Percent rate,

dl./g dl./g. min. Quality F. kw. ML4 Index swell gJmin l. 2 2. 2 4.5Good 255 3. 7 59 16 24 111 2. 0 3. 6 4. 5 Fair 285 4. 8 86 12. 5 60 1143. 8 6. 7 31 Crumbled- 255 3. 7 8 80 80 4.5 7.3 43 do 245 3.6 72 7 34 84TABLE lIL-PRODUCT PROPERTIES OF TREAD STOCK We claim as our invention:

1. The process for the preparation of a branched conjugated dienehomopolymer which consists essentially of polymerizing a conjugateddiene having 4-5 carbon atoms per molecule at 20100 C. in the presenceof an alkyl mono lithium initiator to form a linear polydienyl lithiumfirst polymer having an intrinsic viscosity of 1.02.5 dl./ g. measuredin toluene at 30 C., and reacting the first polymer for 0.1-4 hours at20-80 C. with 0.5-1.5 equivalents per equivalent of lithium, of adiester of a monohydric alcohol and a dicarboxylic acid, whereby abranched polymer having an intrinsic viscosity between 2.0 and 3.75 dl./g. measured in toluene at 30 C. is formed.

2. A process according to claim 1 wherein the diene is isoprene.

3. A process according to claim 2 wherein the coupling compound is adiester of a fatty alcohol and an aliphatic dicarboxylic acid.

4. A process according to claim 3 wherein the branched polymer has anintrinsic viscosity of 2.2-3.6.

References Cited UNITED STATES PATENTS 3,135,716 6/1964 Uraneck et al.26094.2X 3,281,383 10/1966 Zelinski et al. 26094.2X

10 JOSEPH L. SCHOFER, Primary Examiner W. F. HAMROCK, Assistant ExaminerU.S. Cl. X.R.

